1. Field of the Invention
The present invention relates to a low-pressure mercury vapor discharge lamp arranged to have a folded portion in the middle of a discharge passage and two end portions thereof oriented in the same direction, and is turned on such that the portions are oriented in a direction where it will or will not be affected by gravity. More particularly, invention relates to a low-pressure mercury vapor discharge lamp which preferably starts and maintains a high luminous efficacy even if the direction of the portions, ambient temperature, etc. are changed when the lamp is turned on.
2. Description of the Prior Art
A conventional compact fluorescent lamp is arranged such that two end portions of a discharge passage are oriented in the same direction and at least one folded portion is oriented in the opposite direction. This lamp has a drawback that mercury vapor pressure in a tube rises too high at a high temperature.
A H-shaped type fluorescent lamp such as disclosed in Japanese Patent Laid-Open No. 55-133744 is arranged such that middle portions of two straight tubes are joined to each other through a connecting tube section to form a H-shaped folded portion in which a low temperature region is formed in the end portion of the H-shaped tube to condense excessive mercury so as to control the vapor pressure in the tube.
Furthermore, a fluorescent lamp such as disclosed in Japanese Patent Laid-Open No. 57-174846 is arranged such that a middle portion of a straight tube is bent to form a U-shaped folded portion and inside diameters of the straight tube, the summit portion of a bent section and a portion on the way of the bent section having dimensions in D.sub.1, D.sub.2 and D.sub.3 respectively, satisfies .ltoreq.D.sub.2 &lt;D.sub.3 and a low temperature region is formed in an inner surface of an outer angle section of the portion along the bent section to condense excessive mercury so as to control the vapor pressure in the tube.
In these conventional lamps, owing to natural cooling, the temperatures of low temperature regions are different from each other in the case of orienting the folded portion in a direction where it is not affected by gravity (This is called the "base down state" hereinafter.) and in the case of orienting the folded portion in a direction where it is affected by gravity (This is called the "base up state" hereinafter.), even if the ambient temperatures are the same as each other. Furthermore, when the lamp is turned on in the base down state, droplets of condensed mercury may be dropped to an electrode to make the brightness fluctuate and damage the electrode.
As for the solution to the above mentioned problems, such as disclosed in Japanese Patent Laid-Open No. 60-225346, a low-pressure mercury vapor discharge lamp has been developed which employs amalgam for controlling the mercury vapor pressure so that droplets of condensed mercury are not dropped even if the lamp is turned on in the base down state. In the above mentioned discharge lamp, since mercury is tightly condensed in the low temperature region, amalgam in which mercury is more tightly condensed than that (amalgam capable of strongly absorbing mercury vapor) must be employed. Owing to that, on the contrary, drawbacks such mercury not being discharged sufficiently, the lamp not preferably starting or not turning on, and so-called black shade (i.e., a film of mercury compound forming on a glass wall of a tube), etc. are yielded. If amalgam in which mercury is not tightly condensed is employed, mercury is condensed in the above mentioned low temperature region in the case of the base down state. This cannot solve the problem that droplets of condensed mercury drop.